Lithium ion secondary batteries have characteristics such as compact size, light weight, high energy-density, and the ability to be repeatedly charged and discharged, and are used in a wide variety of applications.
A lithium ion secondary battery has generally includes electrodes (a positive electrode and a negative electrode) and a separator that separates a positive electrode and a negative electrode to prevent a short circuit between the positive electrode and the negative electrode. Further, an electrode for a lithium ion secondary battery normally includes a current collector and an electrode mixed material layer formed on the current collector. The electrode mixed material layer is formed by using a slurry composition that is obtained by dispersing, for example, an electrode active material, a binder composition including a binding material, or the like, in a dispersion medium.
As a binder composition used for forming an electrode mixed material layer for a lithium ion secondary battery, a binder composition that includes, as a binding material, a mixture of a fluorine-based polymer such as a polyvinylidene fluoride and hydrogenated nitrile rubbers is used (see PTL 1, for example).
In a lithium ion secondary battery, insertion and desorption of lithium ions are performed for each of a positive electrode and a negative electrode while the battery is charged and discharged, and thus expansion and contraction of the electrodes occurs. Expansion and contraction during insertion and desorption of lithium ions are especially large for a negative electrode. Thus, when charging and discharging are repeated to an electrode, in particular to a negative electrode, of a lithium ion secondary battery over a long period of time, the electrode itself expands and the structure of the electrode is destroyed due to an excessive internal stress, which causes a problem of reduction in a battery performance of a lithium ion secondary battery.
To the aforementioned problem, for example, PTL 2 proposes a technique in which a polyimide having specific mechanical properties is used as a binding material for a negative electrode to absorb and ease expansion and contraction of a negative electrode active material so that degradation of battery performance is suppressed.